Friday, May 11, 2012

How placement of impervious cover in a watershed affects the health of the watershed?

Does the spatial arrangment of Impervious cover matter in relationship to the watershed's outlet or the watershed's streams?

Study Site:

The Analysis Plan with the starting inputs of a 5 meter DEM, NHD Flowlines, delineated watersheds with the outlet points, and an impervious surface cover raster.

With over 2000 watersheds in the state the analysis required a model created in ArcGIS's Model Builder to iterate through all of the watersheds to create a set of Euclidean distance to the outlet rasters and a set of Euclidean distance to the streams rasters.

A second model was necessary to multiply the two sets of rasters by the impervious surface cover raster.

Each of the cells in each watershed would be weighted inversely to their distance to the outlet points and to the streams.

In Watershed Sample ID: 1000

The unweighted percentage of impervious surface cover was 2.91%

The IDW value for Distance to the Outlet was 6.72%

The IDW value for Distance to the Streams was 2.74%

The shows that the impervious surface cover is clustered closer to the outlet. It also shows that the impervious surface cover is clustered slightly farther away from the streams than an even distribution would be.

Using a sample of 15 watersheds the IDW value for Distance to the Outlet was on average 15.86% higher than the unweighted percentages. The IDW value for Distance to the Streams was on average 3.19% lower than the unweighted percentages.

In the future other measures of distance such as flow length and flow gradient will be used to see if they produce a better result than Euclidean distance. In addition, the results of this study need to be compared to an indicator of watershed health to see if any trends develop.